Bicycles provided with devices known as dynamos in order to convert mechanical energy, normally generated by the rotation of the front wheel, into electric energy, for example for supplying the lamps of the bicycle, are known.
The standard dynamos comprise a cylindrical case which houses inside an alternator, the shaft thereof projects from the case and is rotated by a contact element which in turn contacts the bicycle wheel along the circumference thereof, and consequently rolls on the same. The current produced by the alternator is converted in continuous current, for example by a commutator, and then supplies a light of the bicycle, for example.
Moreover, dynamos of the so-called “hub dynamo” type have been proposed which are directly attached to the hub of a bicycle wheel and rotate integrally with it around the hub. Such hub-dynamos comprise inside an electric generator which produces an alternate current due to the wheel rotation, this current then is used for supplying the bicycle lights or possibly also external devices, such as for example a smartphone. The hub-dynamos, in comparison with the standard dynamos, generally generate a smaller rolling resistance and therefore require less effort to a cyclist. However, also these latter generate a not negligible rolling resistance, proportional to the wheel speed and moreover increasing as a function of the number of loads supplied by it.